Your only likely option is Thermadyne dealer. I've got the same machine and for the last five years not spotted any cheap Chinese knock offs.
The genuine parts aren't dear though. I bought a load of spares recently : 5 tips 5 electrodes and a spare shroud body for I think was around 20 quid
I originally purchased my plasma from Pennine welding in Leeds who were then bought out by IWS. I've got parts from the Doncaster branch no problem although it it would be worthwhile phoning first in case they need to order the bits in.